As a result of the constantly increasing integration density in the semiconductor industry, photolithographic masks or templates of the nanoimprint lithography have to project smaller and smaller structures onto a photosensitive layer, i.e., a photoresist dispensed on wafers. In order to fulfil this demand, the exposure wavelength of photolithographic masks has been shifted from the near ultraviolet across the mean ultraviolet into the far ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers. As a consequence, the manufacturing of photolithographic masks with increasing resolution is becoming more and more complex, and thus more and more expensive as well. In the future, photolithographic masks will use significantly smaller wavelengths in the extreme ultraviolet (EUV) wavelength range of the electromagnetic spectrum (approximately 13.5 nm).
Photolithographic masks have to fulfil highest demands with respect to transmission homogeneity, planarity, pureness and temperature stability. In order to fabricate photolithographic masks with a reasonable yield, defects or errors of masks have to be corrected at the end of the manufacturing process. Various types of errors of photolithographic masks and methods for their corrections are described in the U.S. provisional applications U.S. 61/324,467, U.S. 61/351,056 and U.S. 61/363,352, which are herein incorporated by reference in their entirety. A new type of lithography is the nanoimprint technique in which pattern elements are transferred via a polymer layer to an underlying wafer. Since the reproduction scale of pattern elements in a template for the nanoimprint lithography is 1:1, high demands are made to a template with respect to admissible errors. Thus, defective templates also have to be corrected whenever possible.
Femtosecond light pulses of a laser source can be used to correct errors of photolithographic masks and templates for the nanoimprint lithography. For this purpose, the laser source applies a huge local energy density on the transparent material of a substrate of a photolithographic mask or of a template which leads to a local melting of the transparent material. This local melting induces a local variation of the density of the substrate or of the template material. A local density variation is in the following also called a pixel. The introduction of a local density variation by locally applying the laser beam on the material is in the following denoted as the writing of pixels in the transparent material.
The generation of pixels in a transparent material by high intensity femtosecond light pulses induces a local nonlinear optical process at the interaction zone of the photons of the light pulses with the electrons of the material. The introduction of a plurality of pixels in the transparent material results in locally varying displacements of pattern elements arranged on a surface of the transparent material. Moreover, the writing of pixels in a transparent material leads to a second effect in the material as the pixels locally modify the optical transmission of the transparent material.
The document DE 10 2006 054 820 A1 phenomenologically describes the effects of introducing a local density variation in a substrate of a photolithographic mask on the optical transmission of the substrate and of the displacement of pattern elements arranged on the surface of the substrate. The parameters used to differently influence the optical absorption and the displacement of pattern elements are the pixel size and the pixel density. The pixel size is a function of the absorbed optical dose, i.e., of the number of photons locally applied to the material. For the correction of errors of photolithographic masks, the above mentioned document proposes to set-up a library for the arrangements of pixels used for desired corrections of displacement errors.
Thus, the approach of DE 10 2006 054 820 A1 allows to correct some of the errors of photolithographic masks. However, this approach requires some efforts. It is a major drawback of the discussed approach that it does not provide a quantitative description between the effects caused by the laser beam in the transparent material and the parameters of the laser beam used for the error correction.
It is therefore one object of the present invention to provide a method and an apparatus for determining parameters of a laser beam used for the correction of errors of a transparent material.